To complete a well, one or more formation zones adjacent a wellbore are perforated to allow fluid from the formation zones to flow into the well for production to the surface or to allow injection fluids to be applied into the formation zones. A perforating gun string may be lowered into the well and the guns fired to create openings in casing and to extend perforations into the surrounding formation.
The explosive nature of the formation of perforation tunnels shatters sand grains of the formation. A layer of “shock damaged region” having a permeability lower than that of the virgin formation matrix may be formed around each perforation tunnel. The process may also generate a tunnel full of rock debris mixed in with the perforator charge debris. The extent of the damage, and the amount of loose debris in the tunnel, may be dictated by a variety of factors including formation properties, explosive charge properties, pressure conditions, fluid properties, and so forth. The shock damaged region and loose debris in the perforation tunnels may impair the productivity of production wells or the injectivity of injector wells.
One popular method of obtaining clean perforations is underbalanced perforating. The perforation is carried out with a wellbore pressure lower than the formation pressure. After the perforations are created fluid initially flows from the formation through the tunnels removing some of the debris. However, underbalance perforating may not always be effective and may be expensive and unsafe to implement in certain downhole conditions. For example, when reservoir pressure is high and the formation matrix is weak this underbalanced pressure differential may result in collapse of the perforation tunnel and/or excessive sand production.
It is common practice to use a well fluid or mud to create a hydrostatic head in the wellbore. This well fluid can be weighted so as to control to an extent the static pressure differential between the formation pressure and the wellbore pressure. Thus based on the formation characteristics the wellbore fluid may be weighted to create a static underbalance condition wherein the wellbore pressure is less than the formation pressure, a balanced static condition wherein the wellbore pressure and formation pressure are equal, and an overbalance static condition wherein the wellbore pressure is greater than the formation pressure.
Prior art perforation operations often result in damage to the formation which has to be remedied to proceed with production or injection operations. This damage is often caused by utilization of a fluid that allows an excessive dynamic pressure imbalance between the formation and the wellbore proximate the firing of the perforating gun.
When a hydrocarbon bearing reservoir is perforated with shaped charges, several events may occur to cause damage to the formation and impair productivity. Permeability may be reduced due to crushing of the rock around the perforation cavity or by blocking of the perforation tunnel by loose fill or debris, which is created during the penetration of the formation by the shaped charge jet. During an overbalanced state well fluid may enter the tunnels and deposit additional debris. This debris is often difficult to remove from the perforation tunnels with the prior art wellbore fluids. The perforation tunnels created may collapse due to dynamic pressure changes during the perforation process defeating the process or decreasing formation-wellbore communication. Additionally, when the gun is fired with conventional fluids the rapid changes in the wellbore pressure results in “gun jumping.” This lead to damage to the downhole tools, perforating gun, and packers.
Additionally, in prior art completions of wellbores the perforation of the wellbore is an isolated step in the completion of a well. After perforating it may be desired to go back into the wellbore and flow a viscous fluid to remove particles from the formation and the wellbore to facilitate production from the well. It may also be desired to go back into the well and inject a fluid that reacts with the desired formation to clean the perforations and increase production.
Therefore, it is a desire to provide a system for controlling the pressure transient during perforation operations that addresses the shortcomings of the prior perforating systems. It is a further desire to provide a perforating fluid that reduces the damage to a reservoir adjacent a wellbore. It is a still further desire to provide a pressure transient control system that utilizes a perforating fluid to control the dynamic pressure transient during the perforation operation. It is a still further desire to provide a pressure transient control system that utilizes manipulation of the perforating gun to control the dynamic pressure transient. It is a still further desire to provide a wellbore fluid that may aid in the removal of fluid communication barriers between the wellbore and adjacent formation.